And We granted him his brother Harun, (also) a Prophet, out of Our mercy.
This means, "We responded to his request and his plea on behalf of his brother and We made him a Prophet as well."
This is as Allah says in another Ayah,
وَأَخِى هَـرُونُ هُوَ أَفْصَحُ مِنِّى لِسَاناً فَأَرْسِلْهِ مَعِىَ رِدْءاً يُصَدِّقُنِى إِنِّى أَخَافُ أَن يُكَذِّبُونِ
And my brother Harun he is more eloquent in speech than me;so send him with me as a helper to confirm me. Verily, I fear that they will belie me. (28;34)
Also, Allah said,
قَدْ أُوتِيتَ سُوْلَكَ يمُوسَى
(Allah said;) "You are granted your request, O Musa." (20;36)
He also said,
وَيَضِيقُ صَدْرِى وَلَا يَنطَلِقُ لِسَانِى فَأَرْسِلْ إِلَى هَـرُونَ
وَلَهُمْ عَلَىَّ ذَنبٌ فَأَخَافُ أَن يَقْتُلُونِ
So send for Harun. And they have a charge of crime against me, and I fear they will kill me. (26;13-14)
Because of this, some of the Salaf (predecessors) said,
"No one in this life pleaded on behalf of someone else more than Musa pleaded for his brother to be a Prophet."
Allah, the Exalted said,
وَوَهَبْنَا لَهُ مِن رَّحْمَتِنَا أَخَاهُ هَارُونَ نَبِيًّا
And We granted him his brother Harun, (also) a Prophet, out of Our mercy